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To Asa Gray 2 May [1856]1

Down Bromley Kent

May 2d.—

My dear Dr. Gray

I have received your very kind note of Ap. 8th. 2 In truth it is preposterous in me to give you hints;3 but it will give me real pleasure to write to you just as I talk to Hooker, who says my questions are sometimes suggestive owing to my comparing the ranges &c in different Kingdoms of Nature. I will make no further apologies about my presumption; but will just tell you (though I am certain there will be very little new in what I suggest & ask). the points on which I am very anxious to hear about.— I forget whether you include Arctic America, but if so for comparison with other parts of world, I would exclude the Arctic & Alpine-arctic, as belonging to a quite distinct category.4 When excluding the naturalised, I think Decandolle must be right in advising the exclusion, giving list, of plants exclusively found in cultivated land, even when it is not known that they have been introduced by man.—5 I would give list, of temperate plants (if any) found in Eastern Asia, China & Japan, & not elsewhere.— Nothing would give me a better idea of Flora of U.S. than the proportion of the genera to all the genera, which are confined to America; & the proportion of genera, confined to America & Eastern Asia with Japan; the remaining genera wd. be common to America & Europe & rest of world; I presume it wd. be impossible to show any especial affinity in genera (if ever so few) between America & Western Europe.6 America might be related to Eastern Asia (always excluding Arctic forms) by a genus having the same species confined to these two regions; or it might be related by the genus having different species, the genus itself not being found elsewhere. The relation of the genera (excluding identical species) seems to me a most important element in geographical distribution often ignored, & I presume of more difficult application in plants than in animals, owing to the wider ranges of plants; but I find in N. Zealand (from Hooker) that the consideration of genera with representative species tells the story of relationship even plainer than the identity of the species with different parts of the world.—7

I should like to see the genera of the U. States, say 500 (excluding Arctic & Alpine) divided into 3 classes, with the proportions given, thus, 100/500 American genera 200/500 Old World genera, but not having any identical species in common.— 200/500 old world genera, but having some identical species in common: supposing that these 200 genera included 600 U.S. plants; then the 600 would be the denominator to the fraction of the species in common to the Old World.—8 But I am running on at a foolish length.—

There is an interesting discussion in Decandolle (about p. 503 to 514) on the relation of the size of Families to the average range of the individual species; I cannot but think from some facts which I collected long before De Candolle appeared,9 that he is on wrong scent in having taken Families (owing to their including too great a diversity in the constitution of the species), but that if he had taken Genera, he would have found that the individual species in large genera range over a greater area than do the species in small genera:10 I think if you have materials that this would be well worth working out; for it is a very singular relation.—11

With respect to naturalised plants; are any social with you, which are not so in their parent country?12 I am surprised that the importance of this, has not more struck Decandolle. Of these naturalised plants are any or many more variable in your opinion, than the average of your U.S. plants: I am aware how very vague this must be: but De Candolle has stated that the naturalised plants do not present varieties;13 but being very variable & presenting distinct varieties seems to me rather a different case: if you would kindly take the trouble to answer this question, I shd. be very much obliged, whether or no, you will enter on such points in your Essay.—

With respect to such plants, which have their southern limits within your area, are the individuals ever or often stunted in their growth or unhealthy: I have in vain endeavoured to find any Botanist who has observed this point; but I have seen some remarks by Barton on the trees in U.S.14 Trees seem in this respect to behave rather differently from other plants.—

It would be a very curious point, but I fear you would think it out of your Essay, to compare the list of European plants in Tierra del Fuego (in Hooker)15 with those in N. America; for without multiple creation, I think we must admit that all now in T. del Fuego, must have travelled through N. America, & so far they do concern you.—

The discussion on Social plants (vague as the term & facts are) in De candolle strikes me as the best, which I have ever seen:16 two points strike me as eminently remarkable in them; that they should ever be social close to their extreme limits; & secondly that species having an extremely confined range, yet shd. be social where they do occur: I shd. be infinitely obliged for any cases either by letter or publickly on these heads, more especially in regard to a species remaining or ceasing to be social, on the confines of its range.

There is one other point, on which I individually shd. be extremely much obliged, if you could spare the time to think a little bit & inform me; viz whether there are any cases of the same species being more variable in U.S. than in other countries in which it is found: or in different parts of U.S. Wahlenberg says generally that the same species in going S. becomes more variable than in extreme north.17 Even still more am I anxious to know whether any of the genera, which have most of their species horribly variable (as Rubus or Hieracium &c) in Europe, or other part of world, are less variable in U. States: or the reverse case, whether you have any odious genera with you, which are less odious in other countries. Any information on this head, would be a real kindness to me.—

I suppose your Flora is too great; but a simple list in close columns in small type of all the species, genera, & Fams., each consecutively numbered, has always struck me as most useful; & Hooker regrets that he did not give such list in introduction to N. Zealand & other Flora.—

I am sure I have given you a larger dose of questions than you bargained for, & I have kept my word & treated you just as I do Hooker. Nevertheless if anything occurs to me during the next two months, I will write freely, believing that you will forgive me & not think me very presumptuous.

My dear Dr. Gray | Your’s very sincerely | Charles Darwin

I have reread this letter & it really is not worth sending, exceptfor my own sake.: I see I forgot in beginning to state that it appeared to me that the 6 heads of your Essay included almost every point which could be desired, & therefore that I had little to say.—

How well De Candolle shows the necessity of comparing nearly equal areas for proportion of Families!18

Gray was preparing the first part of his paper ‘Statistics of the flora of the northern United States’ (A. Gray 1856–7). In the published version, Gray acknowledged, without mentioning his name, CD’s contribution to the work: ‘While engaged in the preparation of a second edition of the Manual of the Botany of the Northern United States, I was requested by an esteemed correspondent, upon whose judgment I place great reliance, to exhibit, in a compendious and convenient form, the elements of flora I was occupied with.’ (A. Gray 1856–7, p. 204).

Gray attempted a ‘Comparison of the flora of the northern United States with that of Europe in respect to the similar or related species’, in spite of the difficulties such an enterprise entailed ‘owing to the impossibility of estimating the degrees of resemblance among species, or at least of expressing them in any precise or definite way, or bringing shades of difference to any common standard.’ (A. Gray 1856–7, p. 78).

Gray stated that he found this comparison problematic. He therefore settled on taking species rather than genera for his comparison and did not attempt ‘anything beyond an enumeration’ of species in the three classes (A. Gray 1856–7, pp. 80–3).

Bound in CD’s copy of A. de Candolle 1855, 1: pp. 528–9, is a note, dated 1 May 1856, made by CD when studying Alphonse de Candolle’s discussion of the ‘area of species according to families’ (A. de Candolle 1855, 1: 501–32). After working through his own calculations made on data for the Cape flora of South Africa, CD concluded by stating: ‘So that nothing can be inferred safely from these results. Families [’Genera‘ del] being too large.—’ See J. Browne 1980.

Gray followed this suggestion explicitly. After mentioning that Candolle’s calculations were founded upon families, he stated: ‘To be of any value, at least upon our limited scale, the comparison should be made with genera, as Mr. Darwin suggests; and from some investigations of his own, this sagacious naturalist inclines to think that species in large genera range over a wider area than the species of small genera do.’ (A. Gray 1856–7, p. 77). For the most part, Gray’s calculations tended to confirm CD’s generalisation, particularly when the woody plants were examined, although Gray noted that the converse of the proposition ‘does not tell in the same way’ (A. Gray 1856–7, p. 381).

Gray addressed this question only indirectly. As he stated, ‘properly to discuss this and kindred topics would require a great amount of detailed investigation, and would expand these articles into a treatise.’ (A. Gray 1856–7, p. 389). CD had explained his interest in this point in his letter to J. D. Hooker, 14 November [1855] (Correspondence vol. 5).

Summary

Suggests affinities of the U. S. flora that he considers would be worth investigating. Wants to know the ranges of species in large and small genera.

Questions AG on naturalised plants; whether any are social in U. S. which are not so elsewhere and how variable they are compared with indigenous species. Would like to know of any differences in the variability of species at different points of their ranges and also the physical states of plants at the extremes of their ranges.